FIG. 1 is a diagrammatic view of a conventional steam turbine rotor assembly 100 including a disc 102 and a shaft 104. For simplicity, the rotor assembly 100 includes only a single disc and the blades are not shown. The disc 102 includes a rim region 106, a web region 107, and a bore region 108. The disc 102 may be a shrink-fitted disc, which is a disc that is specially processed to provide compressive bore/shaft forces that bind the disc to the shaft.
A defective region 110, for example, a crack, is shown on the disc 102. The defective region 110 may be caused by several failure mechanisms, such as stress corrosion cracking, which occur during the operation of the rotor assembly 100 in a turbine. In a conventional mono-block rotor repair welding technique, the disc is repaired by arc welding the defective region 110 to form a weld 112. During the welding process, a heat-affected zone (HAZ) 114 is formed in a region of the disc that is metallurgically affected by the heat generated by the welding. Next, the disc undergoes a post-weld heat treatment (PWHT) to temper and stress-relieve the HAZ 114 and secondarily, the weld 112. During PWHT, the repaired disc is heated, typically using resistance heating.
The conventional repair technique just described, however, is not very suitable for repairing shrink-fitted discs. Shrink-fitted discs must first be removed from the shaft before the repairs are performed and then refitted onto the shaft after the repairs are completed. The disc must be removed from the shaft to prevent the PWHT from relaxing the disc's shrink-fitted forces that hold the disc on the shaft. In addition, the disc must be removed to prevent the PWHT from overheating the shaft and thus causing it to bow. Due to the difficulties and time required to remove and refit shrink-fitted discs, it has not been considered cost-effective to repair damaged shrink-fitted discs. As a result, defective shrink-fitted discs have been discarded rather than repaired--a rather expensive procedure.
In view of the shortcomings of conventional repair techniques, it is an object of the present invention to provide a method for repairing a shrink-fitted turbine disc without removing the disc from the shaft.